There are several WAL-related configuration parameters that
affect database performance. This section explains their use.
Consult Chapter 18 for general
information about setting server configuration parameters.

Checkpoints are points in the
sequence of transactions at which it is guaranteed that the heap
and index data files have been updated with all information
written before the checkpoint. At checkpoint time, all dirty data
pages are flushed to disk and a special checkpoint record is
written to the log file. (The changes were previously flushed to
the WAL files.) In the event
of a crash, the crash recovery procedure looks at the latest
checkpoint record to determine the point in the log (known as the
redo record) from which it should start the REDO operation. Any
changes made to data files before that point are guaranteed to be
already on disk. Hence, after a checkpoint, log segments
preceding the one containing the redo record are no longer needed
and can be recycled or removed. (When WAL archiving is being done, the log segments
must be archived before being recycled or removed.)

The checkpoint requirement of flushing all dirty data pages to
disk can cause a significant I/O load. For this reason,
checkpoint activity is throttled so I/O begins at checkpoint
start and completes before the next checkpoint starts; this
minimizes performance degradation during checkpoints.

The server's checkpointer process automatically performs a
checkpoint every so often. A checkpoint is created every checkpoint_segments
log segments, or every checkpoint_timeout
seconds, whichever comes first. The default settings are 3
segments and 300 seconds (5 minutes), respectively. In cases
where no WAL has been written since the previous checkpoint, new
checkpoints will be skipped even if checkpoint_timeout has
passed. If WAL archiving is being used and you want to put a
lower limit on how often files are archived in order to bound
potential data loss, you should adjust archive_timeout parameter
rather than the checkpoint parameters. It is also possible to
force a checkpoint by using the SQL command CHECKPOINT.

Reducing checkpoint_segments and/or
checkpoint_timeout causes checkpoints to
occur more often. This allows faster after-crash recovery (since
less work will need to be redone). However, one must balance this
against the increased cost of flushing dirty data pages more
often. If full_page_writes
is set (as is the default), there is another factor to consider.
To ensure data page consistency, the first modification of a data
page after each checkpoint results in logging the entire page
content. In that case, a smaller checkpoint interval increases
the volume of output to the WAL log, partially negating the goal
of using a smaller interval, and in any case causing more disk
I/O.

Checkpoints are fairly expensive, first because they require
writing out all currently dirty buffers, and second because they
result in extra subsequent WAL traffic as discussed above. It is
therefore wise to set the checkpointing parameters high enough
that checkpoints don't happen too often. As a simple sanity check
on your checkpointing parameters, you can set the checkpoint_warning
parameter. If checkpoints happen closer together than checkpoint_warning seconds, a message will be
output to the server log recommending increasing checkpoint_segments. Occasional appearance of such
a message is not cause for alarm, but if it appears often then
the checkpoint control parameters should be increased. Bulk
operations such as large COPY transfers
might cause a number of such warnings to appear if you have not
set checkpoint_segments high enough.

To avoid flooding the I/O system with a burst of page writes,
writing dirty buffers during a checkpoint is spread over a period
of time. That period is controlled by checkpoint_completion_target,
which is given as a fraction of the checkpoint interval. The I/O
rate is adjusted so that the checkpoint finishes when the given
fraction of checkpoint_segments WAL
segments have been consumed since checkpoint start, or the given
fraction of checkpoint_timeout seconds
have elapsed, whichever is sooner. With the default value of 0.5,
PostgreSQL can be expected to
complete each checkpoint in about half the time before the next
checkpoint starts. On a system that's very close to maximum I/O
throughput during normal operation, you might want to increase
checkpoint_completion_target to reduce
the I/O load from checkpoints. The disadvantage of this is that
prolonging checkpoints affects recovery time, because more WAL
segments will need to be kept around for possible use in
recovery. Although checkpoint_completion_target can be set as high as
1.0, it is best to keep it less than that (perhaps 0.9 at most)
since checkpoints include some other activities besides writing
dirty buffers. A setting of 1.0 is quite likely to result in
checkpoints not being completed on time, which would result in
performance loss due to unexpected variation in the number of WAL
segments needed.

There will always be at least one WAL segment file, and will
normally not be more than (2 + checkpoint_completion_target) * checkpoint_segments + 1 or checkpoint_segments + wal_keep_segments
+ 1 files. Each segment file is normally 16 MB (though this size
can be altered when building the server). You can use this to
estimate space requirements for WAL. Ordinarily, when old log segment files
are no longer needed, they are recycled (renamed to become the
next segments in the numbered sequence). If, due to a short-term
peak of log output rate, there are more than 3 * checkpoint_segments + 1 segment files, the
unneeded segment files will be deleted instead of recycled until
the system gets back under this limit.

In archive recovery or standby mode, the server periodically
performs restartpoints which are similar
to checkpoints in normal operation: the server forces all its
state to disk, updates the pg_control
file to indicate that the already-processed WAL data need not be
scanned again, and then recycles any old log segment files in
pg_xlog directory. A restartpoint is
triggered if at least one checkpoint record has been replayed and
checkpoint_timeout seconds have passed
since last restartpoint. In standby mode, a restartpoint is also
triggered if checkpoint_segments log
segments have been replayed since last restartpoint and at least
one checkpoint record has been replayed. Restartpoints can't be
performed more frequently than checkpoints in the master because
restartpoints can only be performed at checkpoint records.

There are two commonly used internal WAL functions: LogInsert and LogFlush. LogInsert is used to place a new record into
the WAL buffers in shared
memory. If there is no space for the new record, LogInsert will have to write (move to kernel
cache) a few filled WAL
buffers. This is undesirable because LogInsert is used on every database low level
modification (for example, row insertion) at a time when an
exclusive lock is held on affected data pages, so the operation
needs to be as fast as possible. What is worse, writing
WAL buffers might also force
the creation of a new log segment, which takes even more time.
Normally, WAL buffers should
be written and flushed by a LogFlush request, which is made, for the most
part, at transaction commit time to ensure that transaction
records are flushed to permanent storage. On systems with high
log output, LogFlush requests might
not occur often enough to prevent LogInsert from having to do writes. On such
systems one should increase the number of WAL buffers by modifying the configuration
parameter wal_buffers. When
full_page_writes
is set and the system is very busy, setting this value higher
will help smooth response times during the period immediately
following each checkpoint.

The commit_delay
parameter defines for how many microseconds the server process
will sleep after writing a commit record to the log with
LogInsert but before performing a
LogFlush. This delay allows other
server processes to add their commit records to the log so as to
have all of them flushed with a single log sync. No sleep will
occur if fsync is
not enabled, or if fewer than commit_siblings
other sessions are currently in active transactions; this avoids
sleeping when it's unlikely that any other session will commit
soon. Note that on most platforms, the resolution of a sleep
request is ten milliseconds, so that any nonzero commit_delay setting between 1 and 10000
microseconds would have the same effect. Good values for these
parameters are not yet clear; experimentation is encouraged.

The wal_sync_method
parameter determines how PostgreSQL will ask the kernel to force
WAL updates out to disk. All
the options should be the same in terms of reliability, with the
exception of fsync_writethrough, which
can sometimes force a flush of the disk cache even when other
options do not do so. However, it's quite platform-specific which
one will be the fastest; you can test option speeds using the
pg_test_fsync module. Note that this
parameter is irrelevant if fsync has
been turned off.

Enabling the wal_debug
configuration parameter (provided that PostgreSQL has been compiled with support
for it) will result in each LogInsert and LogFlushWAL
call being logged to the server log. This option might be
replaced by a more general mechanism in the future.